This invention relates generally to heat exchanger tubes of the type used in shell and tube type heat exchangers. More particularly, the invention relates to a tube for use in an application such as a condenser for an air conditioning system.
A shell and tube type heat exchanger has a plurality of tubes contained within a shell. The tubes are usually arranged to provide a multiplicity of parallel flow paths for one of two fluids between which it is desired to exchange heat. The tubes are immersed in a second fluid that flows through the heat exchanger shell. Heat passes from the one fluid to the other fluid by through the walls of the tube. In one typical application, in an air conditioning system condenser, a cooling fluid, usually water, flows through the tubes of the condenser. Refrigerant flows through the condenser shell, entering as a gas and leaving as a liquid. The heat transfer characteristics of the individual tubes largely determines the overall heat transfer capability of such a heat exchanger.
There are a number of generally known methods of improving the efficiency of heat transfer in a heat exchanger tube. One of these is to increase the heat transfer area of the tube. In a condensing application, heat transfer performance is improved by maximizing the amount of tube surface area that is in contact with the fluid.
One of the most common methods employed to increase the heat transfer area of a heat exchanger tube is by placing fins on the outer surface of the tube. Fins can be made separately and attached to the outer surface of the tube or the wall of the tube can be worked by some process to form fins on the outer tube surface.
Beside the increased heat transfer area, a finned tube offers improved condensing heat transfer performance over a tube having a smooth outer surface for another reason. The condensing refrigerant forms a continuous film of liquid refrigerant on the outer surface of a smooth tube. The presence of the film reduces the heat transfer rate across the tube wall. Resistance to heat transfer across the film increases with film thickness. The film thickness on the fins is generally lower than on the main portion of the tube surface due to surface tension effects, thus lowering the heat transfer resistance through the fins.
It is possible, however, to attain even greater improvement in condensing heat transfer performance from a heat transfer tube as compared to a tube having a simple fin enhancement.